Origin of surface electrical degradation of insulators due to ionic bombardment

Insulator materials required for ITER and beyond must operate in a significant radiation field, extending well beyond the first wall. As a result, these materials will be subjected not only to neutron and gamma irradiation, but also to particle bombardment, due mainly to ionization of the residual gas and acceleration of the resulting ions by local electric fields. A systematic study was carried out on the main insulating candidate materials for ITER (Al2O3, SiO2, BeO, and AlN), in order to assess this potential surface degradation issue, and clarify possible mechanisms. Severe surface optical and electrical degradation has previously been reported for these materials bombarded with H+, D+, and He+ ions, at different energies, temperatures, and dose rates, as well as for electron irradiation. In all cases, dramatic degradation has been found and related to loss of oxygen (nitrogen) from the implanted/irradiated zone due to preferential radiolytic anion sputtering. In the work reported here, a model is presented to explain the surface electrical degradation of insulators under ion bombardment. A general mathematical expression for the evolution of the surface conductivity with time (dose) of an insulator or extrinsic semiconductor subjected to preferential sputtering has been obtained. Very good agreement between the experimental data and the theoretical model has been obtained, and a clear dependence of the sputtering cross-section on the bombarding ion mass has been identified.